Offshore production units which produce hydrocarbons such as oil and natural gas, also produce a range of pollutants along with the hydrocarbons, including sand, stones, water, and contaminants that tend to remain suspended or dissolved in the water. Sand and stones generally can be dumped overboard because they sink to the seafloor, and some contaminants also can be dumped overboard along with water. However, many jurisdictions prohibit dumping into the sea, certain contaminants that may be harmful to sea life or to humans, or dumping water containing a high percent of the contaminants.
Contaminants in hydrocarbons that cannot be dumped overboard can be stored on the production unit in tanks specially used for this purpose. The production units also may include a water treatment plant to enable some of the contaminants to be separated out and dumped along with some of the water, to minimize produced water storage requirements. The production unit also includes storage tanks for storing produced hydrocarbons which the unit offloads to a tanker, or instead, the unit itself may sail away to a distant facility to offload the hydrocarbons before returning. To the extent that tanks on the production unit must be devoted to treating produced water and storing water that cannot be dumped, the production unit can store less hydrocarbons and offloading of hydrocarbons from the unit must occur more frequently which makes production more expensive.
A variety of production units are available, including FPSO (floating production and storage) units, TPL (tension leg platforms), SPAR units (long cylindrical structures) and fixed towers. The amount of water produced usually increases significantly over the life of the field. For example in a certain representative offshore field, the production of water was about 50,000 m3 per day at the start up of hydrocarbon production, but expanded to 150,000 m3 to 300,000 m3 per day during a period of several years after startup. If each unit must be provided with sufficient production water storage capacity to store the maximum amount produced during the life of a field, then the oil storage capacity of the unit will be severely affected. A solution to the problem of disposing of variable large amounts of produced water without requiring very large water storage capacity in the production units, would be of value.